Satellite-supported broadcasting systems provide adequate basic coverage only in rural areas. Therefore, in densely built-up areas, where the reception from satellites is not perfect, terrestrial “re-broadcasting” must be carried out in addition. This means that the satellite signal is received and converted from a fixed receiver directed at the satellite, and is then re-broadcasted from a terrestrial transmitter. Signals from this terrestrial transmitter can then be received by mobile receivers, such as car radios.
For digital broadcasting, pieces of music or speech sequences can be coded, for example, in accordance with ISO MPEG Layer 3. Such reduced redundancy coding limits the considerable quantity of digital information to be transmitted. For example, an MPEG-coded piece is preferably channel-coded in the transmitter, in order to achieve a certain degree of freedom from errors right from the start. Algorithms for error protection include, for example, a Reed-Solomon code and a convolution code. For decoding the convolution code in the receiver symbol-by-symbol MAP is usually used, or the Viterbi algorithm, which works according to the principle of maximum likelihood estimation.
For terrestrial re-broadcasting, larger cities are preferably served by a so-called single frequency network (SFN=Single Frequency Network). This means that areas which cannot be covered by a single transmitter are re-transmitted by means of several transmitters, which transmit the identical signal synchronously.
Implementation of an SFN, as well as error protection codings in the transmitter and the corresponding decodings in the receiver, are well known to experts in this field. With regard to different channel coding possibilities, reference is made to “Channel Coding With Multilevel/Phase Signals” by Gottfried Ungerboeck, IEEE Transactions on Information Theory, volume IT-28, no. 1, pages 55-66, January 1982.
In systems of this type, Multi-Carrier-Modulation (MCM) can be used as the modulation. Multi-Carrier-Modulation can be implemented, for example, by a so-called OFDM-modulation (OFDM=Orthogonal Frequency Division Multiplex).
In OFDM-modulation an OFDM symbol is first formed from a section or block of an input bit sequence. For this, a bit sequence is represented on another sequence of numbers. This type of representation is also known technically as “mapping”. In the simplest case mapping means the combination of two sequential bits of the input sequence in order to obtain a dibit, i.e. a digital word of length two bits. Depending on the number of carriers present, as many digital words are now stored in parallel as there are carriers present. This parallel arrangement corresponds to the formation of a complex spectrum, wherein each digital word (i.e. each dibit in the example) is a complex representation of one carrier for a plurality of carriers. In order to transmit this spectrum, it is transformed into the time domain by means of an inverse Fourier transform, which is produced as a Discrete Fourier Transform (DFT) or as a Fast Fourier Transform (FFT).
The result of the transform of one spectrum from a large number of dibits or information symbols is also known as an MCM-symbol. This MCM-symbol can preferably be extended by one protection interval in the time domain, so that no Inter Symbol Interference (ISS) occurs. Several MCM symbols, between each of which a guard or protection interval is inserted, can be combined to form an MCM frame, which is provided with a synchronisation sequence for synchronisation of the receiver. The MCM frame thus consists of several MCM-symbols, between each of which there is a protection interval, and a synchronisation sequence. Timing of the protection interval should be sufficiently long that, in an SFN system, repeated reception from transmitters other than the nearest located transmitter, which all transmit synchronously at the same frequency, does not lead to loss of data.
With regard to OFDM modulation, reference is made, for example, to the scientific publication “Data Transmission by Frequency-Division Multiplexing Using the Discrete Fourier Transform”, by S. B. Weinstein et al., IEEE Transactions on Communication Technology, volume COM-19, no. 5, pages 628–634, October 1971. With regard to OFDM with channel coding, reference is made, for example, to the scientific publication “COFDM: An Overview” by William Y. Zou et al., IEEE Transactions on Broadcasting, volume 41, no. 1, pages 1-8, March 1995.
The principles of OFDM and channel coding for the OFDM by means of convolution codes, and channel decoding by means of the Viterbi algorithm are well known, and have been described in detail in the publications mentioned. It is therefore not necessary to explain these aspects in detail here. One problem with multi-carrier transmission systems (CM), which also include the OFDM systems, is the fact that during transmission of information over multiple channels many carriers can be (almost) completely subject to fading. Information which is transmitted by means of these carriers is therefore no longer available to the receiver, and can only be recovered (if at all) by efficient channel coding.
Interference of the non-ideal transmission channel can consist, for example, of Additive White Gaussian Noise (AWGN), a time-dependent increased damping of the transmission channel (for example, when driving in the “shadow” of a high-rise building), a frequency-selective transmission channel, i.e. certain frequencies are more strongly damped than other frequencies, or (usually) a combination of the phenomena mentioned. Furthermore, owing to the highly inhomogeneous topology of the transmission channel, i.e. the many buildings in a city, reflections can also take place. As has already been mentioned, under corresponding running time conditions, these lead to constructive, but also to destructive, interferences. This situation becomes more aggravated owing to the fact that, in addition to the multi-channel reception (which exists owing to the different transmission paths), in an SFN-system system-related signals from other transmitters are received, which transmit in synchronisation with a transmitter which is dominant in relation to the receiver. Signals for such broadcast relay transmitters will have longer times of travel to the receiver; however, owing to constructive interferences it is quite possible that their amplitudes will come within the range of the receiver amplitude of the dominating transmitter, particularly if this, for its part, is strongly damped by a destructive interference.
U.S. Pat. No. 4,606,047 relates to a RF communication modem using frequency as well as time diversity for eliminating transmission problems like noise, multiple path transmission etc. A digitally coded signal is sequentially transmitted in five complementary dual tone channels, wherein the first tone of a channel, i.e., the first carrier of a channel carries the actual bit to be transmitted, while the second tone of the channel transmits the complementary state of the first channel. In each channel, the transmit bit and the complementary bit are transmitted simultaneously, wherein the transmission in the five channels takes place in a time staggered manner.
EP 0 572 171 A1 relates to a method and apparatus for providing time diversity for channels, that are affected by multiple path fading. A digital signal is channel-coded to generate one or more symbols. Then, a plurality of symbol copies is made, wherein each copy is weighted by a fixed time-varying function. The weighted symbol copies are processed by means of different transmitting circuits and transmitted by means of antennas connected to respective transmitting circuits. The weighting of the symbols by means of the time-varying function includes changing the amplitude amplification, the phase shift or the amplitude amplification and the phase shift. The weighted copies of a symbol are transmitted simultaneously. So called “deep fades” are overcome by the fact that the weighting using the time-varying signal introduces different phase/amplitude situations. Although, also in this situation, a destructive interference can occur because of the weighting, the interfering signals are changed such that the “deep fades” are no longer “stationary”, but only occur during a certain portion of the time-varying weighting functions.